Unknown ATA chipsets are
supported in PIO modes, and if the standard busmaster DMA registers
are present and contain valid setup, DMA is also enabled, although
the max mode is limited to UDMA33, as it is not known what the
chipset can do and how to program it.

The ata driver
can change the transfer mode and various other parameters when the
system is up and running. See atacontrol(8).

The ata driver
sets the maximum transfer mode supported by the hardware as
default. However the ata driver sometimes warns:
‘‘DMA limited to UDMA33, non-ATA66 cable or
device’’. This means that the ata driver has
detected that the required 80 conductor cable is not present or
could not be detected properly, or that one of the devices on the
channel only accepts up to UDMA2/ATA33.

Unknown ATAPI devices
are initialized to DMA mode if the hw.ata.atapi_dma tunable
is set to 1 and they support at least UDMA33 transfers. Otherwise
they are set to PIO mode because severe DMA problems are common
even if the device capabilities indicate support. You can always
try to set DMA mode on an ATAPI device using atacontrol(8), but be
aware that your hardware might not support it and can
potentially hang the entire system causing data loss.

FILES
/dev/ad*

ATA disk device
nodes

/dev/ar*

ATA RAID device
nodes

/dev/acd*

ATAPI CD-ROM device
nodes

/dev/afd*

ATAPI floppy drive
device nodes

/dev/ast*

ATAPI tape drive device
nodes

/sys/i386/conf/GENERIC

sample generic kernel
config file for ata based systems

NOTES

Please remember that in order to use
UDMA4/ATA66 and above modes you must use 80 conductor
cables. Please assure that ribbon cables are no longer than 45cm.
In case of rounded ATA cables, the length depends on the quality of
the cables. SATA cables can be up to 1m long according to the
specification.

Static device numbering
(enabled with the ATA_STATIC_ID kernel option) reserves a number
for each possibly connected disk, even when not present. This is
useful in hotswap scenarios where disks should always show up as
the same numbered device, and not depend on attach order.